1. Field of the Invention
The present invention relates to a solid state pickup device, and more particularly, to a solid state pickup device implemented as a complementary MOS IC.
2. Background of the Invention
Heretofore, various kinds of solid state pickup devices have employed charge coupled elements (referred to herein as a CCD solid state pickup device) or have employed MOS solid state pickup devices. Both of these types of devices are often designed in conformity with the manufacturing process of n-channel MOS (nMOS) ICs.
The reason for this (i.e. use of nMOS ICs) is attributed to the effective utilization in the art of designing nMOS ICs, which has made a marked progress for designing picture elements of high resolution and sensitivity with relative simplicity.
In the conventional solid state pickup device, however, a photosensitive section where photoelectric conversion elements, such as photo diodes constituting picture elements are disposed are integrally formed in one and the same chip with horizontal and vertical scanning shift registers for reading out a video signal produced in each photoelectric conversion element. Nonetheless, it is still difficult to form, integrally with the photosensitive region, a peripheral circuit, especially, an oscillation circuit for generating clock signals applied to the shift registers, and a processing circuit for performing a correction process based on the video signals thus read out and for color signal isolation.
The reason for the difficulty is that parameters for optimizing the photoelectric conversion characteristics of the photoelectric conversion element being produced, e.g., the impurity concentration of the semiconductor substrate and the depth of the p-n junction of the photoelectric conversion element, are incompatible with the formation of a logic circuit offering superior characteristics. That is, even though improvements in the photoelectric conversion characteristics are attempted, the use of the aforesaid manufacturing parameters for designing a peripheral circuit will result in a low noise margin or a high rate of power consumption.
In consequence, a solid state pickup device equipped with a photosensitive region and a peripheral circuit must be built with different manufacturing parameters as an individual IC device and then wired together on an electric circuit board. However, the above described process of manufacturing solid state pickup devices has caused an obstruction to the miniaturization of electric devices in cameras and so forth.
Moreover, solid state pickup devices equipped with photosensitive sections are manufactured according to the process of designing nMOS ICs, whereas use is often made of complementary MOS (CMOS) ICs capable of providing a wide range of operating voltages with a small power consumption and a high noise margin for the manufacture of peripheral circuits. As a result, the solid state pickup devices and the peripheral circuits require mutual matching.
Therefore, there has been proposed a solid state pickup device whose photosensitive region and peripheral circuit are integrally formed on the same semiconductor chip as a CMOS IC. However, the conventional solid state pickup device as a CMOS IC presents the following problems.
As set forth above, the quality of photoelectric conversion characteristics is dependent on manufacturing parameters including the impurity concentration of the semiconductor substrate and the depth of the p-n junction of the photoelectric conversion element such as a photodiode for use in forming a picture element. Even though he solid state pickup device is made consistent with a CMOS IC, the impurity concentration of the semiconductor substrate should be lower by about an order a magnitude than what is employed in the conventional art of designing CMOS ICs in order to obtain optimum photoelectric conversion characteristics. As is well known, the impurity concentration of the semiconductor substrate is set high to prevent a latch-up phenomenon due to parasitic thyristor current paths. However, if an improvement in photoelectric conversion characteristics is attempted as mentioned above, the latch-up phenomenon will readily occur and will cause internal elements to be damaged.
In addition, the depth of the p-n junction of the photoelectric conversion element is set small than that in the case of the conventional CMOS IC in consideration of color sensitivity, e.g., sensitivity to blue color. Accordingly, the junction depth is unfit for designing the peripheral circuit.
As set forth above, since the manufacturing parameters fit for determining the photoelectric conversion characteristics and the peripheral circuits are related oppositely to each other, it has hardly been possible to manufacture solid state pickup devices offering good quality characteristic as a CMOS IC.